So I fiddled around a little bit with gemming before tonights raid. I downloaded Rawr and tried to understand it, and I think I did. In reality, I prob. Failed.It told me to almost gem pure Stamina with a very few parry/dodge/mastery gems with stamina on the side. Now I dont feel comfortable doing that. From what I can read, I should focus on mastery until I hit a CTC of 102,4(or is it 104,2?). Now I've regemmed entirely and did it like this:

For gemming, I would just optimise manually - MrRobot is great, but it focuses on melee damage reduction when what you want is to get to 102.4% CTC without cutting stamina too much. Have a look at Digren's gear enchancement guide stickied here. I've tended to go for slot bonuses rather than pure mastery all over, but if I could reach CTC cap by gemming alone, I would. (As it is, it requires ditching my stamina trinket which is too steep a price for me - 10k hp for 4% block.)

There is a nuance with balancing parry and dodge (iirc you want the former rating about 155 higher?) but as long as those two stats are fairly well balanced - to limit diminishing returns - it won't matter much. I refer to Mr Robot for ideas on reforging and it usually gives me something that makes sense but I've never looked at if for gemming.